“The fundamental question is can you actually get away with flying IFR, in controlled airspace to or from a 'proper' airport, in a Permit aircraft?”
No, you will get caught. A Europa pilot was recently caught asking for an IFR clearance through EMA (in IMC), the rumour is he is to be prosecuted, but even if this is not the case he was caught. There are some “grey” areas which might allow you to bend the rules. If you build a Falco, you may get away with it as this aircraft was factory built and IFR approved, as well as being kit built, so most airports would assume an IFR arrival in a Falco would be Factory built and OK, but an RV or Europa would not stand a chance in my view.
The LAA were very bullish on getting IFR approval in the medium term, but I am not up to date on this.
Rod1